The present invention relates to an apparatus for optically reproducing the recorded information and, more particularly, to a circuit for reproducing an RF (high frequency) signal in the apparatus for optically reproducing the recorded information.
In the apparatus for optically reproducing the recorded information, the RF signal, a focussing error signal and a tracking error signal are produced on the basis of the reflected light obtained by irradiating a light beam onto the recording surface of an information recording disc (hereinafter, simply referred to as a disc). On the other hand, it has been well known that in order to produce the RF signal and the focussing error signal by a common photo sensing device, what is called a four-split type photo sensing device is used. Namely, the photo sensing surface of this device is divided into four surfaces by a split line in the direction which is parallel with the tangential line of the recording track and by another split line perpendicular to the former split line.
In the RF signal producing circuit using such a four-split type photo sensing device, the light beam for reproduction is focussed onto the recording surface of the disc so as to irradiate as a light beam spot thereto. The reflected light from the recording surface based on the irradiated light beam is led to the four split surfaces of the photo sensing device. The outputs of the surfaces locating on the same side regarding the split line perpendicular to the tangential direction of the track are respectively added to obtain two addition outputs A and B. Further, the addition outputs A and B are added thereby to produce the RF signal.
In the RF signal producing circuit, when the depth d of pit which forms the recording track is smaller than .lambda./4 (.lambda. denotes a wavelength of the light beam), the addition outputs A and B have the same waveform and there is a constant time difference (phase difference) between them. Therefore, the leading and trailing edges of the addition output (the RF signal) of the outputs A and B are dull. The amplitude of the output waveform when the length of pit is short is smaller by a predetermined level than the amplitude of the output waveform when the length is long. Namely, the attenuated amount of the high frequency characteristic which is caused due to the addition appears in the RF signal.
To prevent such a situation, the same applicant as the present invention has already proposed a circuit such that phase adjusting means for making the phases of both addition outputs A and B coincident is provided, the addition outputs A and B are transmitted through the phase adjusting means, the resultant addition outputs A and B whose phases are coincident are added to form the RF signal, thereby making the leading and trailing edges of the RF signal steep and enabling the reduction of the level of the RF signal to be suppressed (refer to Japanese Utility Model Laid-Open No. 60-23932).
However, such a situation occurs only when the depth d of pit is smaller than .lambda./4. When d=.lambda./4, no phase difference occurs between both addition outputs A and B. Therefore, according to such a circuit which is constituted so as to always perform the phase adjustment, an unnecessary phase adjustment is also executed even if the pit whose depth d is equal to .lambda./4.